Hi Joe,

Typically we have found that the Dk or Er is somewhat overstated by the
supplier. We have found the Dk to be closer to 4.0 than 4.5. You may want to
feed real Info from the board into a transposed Equation and back calculate the
Dk or Er value. Of course all this depends on the finished resin to glass ratio
along with the other Significant parameters already discussed by others. No
need to go into those; I'm sure you are aware of them anyway.

Good Luck,

Les Connally
[log in to unmask]

>  From: "Joseph E. J. Duclos Jr." <[log in to unmask]>, on 3/9/98 6:15
>  PM:
>  Hello all, an odd question that has come up in some evaluations:
>
>  I have a  "known good" impedance calculator formula from IPC-2141 that has
>  worked for years on Polyclad 226 material.
>
>  I know my circuit widths, dielectrics, stackup and coupon design are good.
>
>  THE DIFFERENCE tried Polyclad 370 material      Higher Tg (170 vs 140)
>                                                  DK/Er (4.5 on both)
>
>  THE PROBLEM -   my formula says 48-52 ohms
>                          coupons read on 2 different TDRs at 59-63 ohms
>                          One has a Tr @ 30ps the other is newer so probably a
>  quicker Tr
>
>  THE QUESTION(S) 1) Am I in error ?
>                          2) Should I be using a compensation factor ?
>                          3) Do I have incorrect data on the 370 material ?
>                          4) Have the laws of physics forsaken me for some sin
>  against the stray molecular particle ?
>
>  Any help would be greatly appreciated, especially on questions 1,2 & 3.
>
>  JOE SENDZ....
>
>
>
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